This invention relates generally to the top unloading of bulk materials from relatively large cargo containers or vessels, or the transfer of bulk material from a cargo container to a material transport, handling or storage system. More particularly, this invention relates to the unloading of dry powder-like cement or other loose, dry material from a barge or the like. It is especially useful where the container does not have a self-contained unloading means or crane, or is not equipped with a gravity feed system. Moreover, the unloader of the present invention may be portable and self-contained, for operation from work barges, ships, or the like.
Dry cement is a fine powdered material (similar to talc) and is referred to hereinafter as "cement". Its size is very fine, 100 mesh or under; it is relatively free flowing at an angle of repose of 30.degree. to 45.degree.; and it is mildly abrasive. When aerated it becomes fluid-like, but when packed or if it settles, it becomes very dense and difficult to handle. Cement is widely utilized in construction because it has strong adhesive properties after it is combined with water. Once properly mixed, it sets in a few hours and cures or fully hardens over a period of weeks.
Manufactured in large plants, the cement may be transported in bulk quantities by truck, rail or barge. However, due to its physical properties and fickle nature, cement requires a unique handling system. Small quantities may be conveniently unloaded or moved with a pneumatic system, but this has not proven efficient or effective for larger quantities. Thus, mechanical systems have been relied upon. However, due to its powdery nature, the density of cement may vary from a relatively light, fluffy and fluid-like or relatively free flowing aerated mixture of less than fifty pounds per cubic foot, to a heavy, more solid and concentrated mass of over 150 pounds per cubic foot.
Depending upon the conditions encountered upon loading and during transport, the cement may become tightly compacted, requring a mechanical, rather than pneumatic, unloader. In addition, when the cement is being transported, and even during the unloading process, the movement and vibration may cause the cement to settle or compact to its more dense state, causing pneumatic systems to plug and mechanical systems to jam, overload or bind. This usually results in undesireable and expensive down-time, accelerates wear and tear on equipment that is already operating in a harsh environment, and may result in equipment failure. On the other hand, even when the cement is in its fluffy aerated condition, it poses unique handling and transport problems because it is dust-like and requires a closed transport system to minimize pollution, loss, and wear of equipment due to abrasion. This also protects the cement from exposure to elements which may contaminate it, or cause it to absorb moisture and harden. For this latter reason, it is important that the bulk container be emptied completely so that residual cement does not set up and harden each time the container is used.
Other bulk material unloading systems are available and known in the prior art, but most are gargantuan, requiring heavy capacity dockside cranes, ship derricks, or the use of gantry cranes that are supported by the container or barge. Still others are not suitable for handling cement. In addition, some of these unloaders required that the barge or container be moved in order to reach all areas of the interior of the container.